This invention relates to a vapor pressure measuring apparatus.
While the apparatus of the present invention was designed specifically for measuring the vapor pressure of liquid hydrocarbons, it will be appreciated that the apparatus can be used to measure the vapor pressure of a wide variety of liquids, both hydrocarbon and non-hydrocarbon. The analyzer may be used to determine liquid Reid vapor pressure in "gage" or "absolute" units for hydrocarbon liquids as defined in procedure ANSI/ASTM D 1269-79 and vapor pressure of hydrocarbon liquid defined in procedure ANSI/ASTM D 323-79, respectively. Depending on inlet conditions such as pressure and temperature of a sample, the analyzer will measure vapor pressure over a wide range of temperatures thus determining a vapor pressure curve for each sample. The analyzer is intended for operation on a batch basis with time cycles typically in the range of one to three minutes, and can produce repeatable results within the accuracy specified by the above mentioned ANSI/ASTM procedures.
At present, the above mentioned ANSI/ASTM laboratory procedures are commonly used to measure Reid vapor pressure of petroleum products. Both methods are laboratory procedures which require the performance of specific steps to produce predictably accurate results. Both methods require careful handling of samples, preconditioning of sample containers, heat bath systems, and agitation of samples during an approximate thirty minute time period to ensure mixing and vapor separation. The ANSI/ASTM D 323-79 procedure requires an air chamber to be preheated to 100.degree. F. before sampling.
In the past, a variety of methods of measuring vapor pressures have been proposed, including the use of an apparatus consisting of a temperature control bath, a continuous sampling device and two positive displacement pumps of different capacities to cause vapor separation. Vapor pressure is measured in a chamber, the temperature of which is controlled by a bath at 100.degree. F. The device was designed primarily for measuring the vapor pressure of crude oil. In accordance with another method, the vapor pressure of blended gasoline products has been determined by measuring the temperature drop produced by the expansion of a liquid sample from a high to a low pressure under substantially adiabatic conditions. In order to improve accuracy, methods have been devised to remove accumulated fixed gases in the apparatus. Finally, vapor pressure has been measured using an apparatus including a temperature control bath and two sample pumps of different capacities. In essence, a sample is continuously vaporized across an orifice plate into a chamber. The vapor pressure is measured in the chamber by a bellows-type pressure gage employing a Wheatstone Bridge.
The above described methods are reasonably accurate. However, they employ elaborate sampling techniques such as time consuming manual laboratory procedures and the use of two pumps in series to collect samples and ensure vaporization. The laboratory procedure includes special sampling steps and agitation of the liquid to ensure mixing. The continuous sampling device requires a liquid, temperature controlled bath which is difficult to use in industrial applications without special consideration. Bath temperature is difficult to control within .+-./1.5.degree. F. accuracy. The continuous device requires varying levels of calibration to a standard before handling liquids of significantly different composition or physical properties. Moreover, the prior art devices cannot provide vapor pressure values over a wide range of temperatures during each sampling cycle.
The object of the present invention is to overcome the above-defined difficulties encountered with prior art devices by providing an automated apparatus which is relatively simple in terms of design, reliability and accuracy, and which can be used to determine the vapor pressure of a wide variety of hydrocarbon and non-hydrocarbon liquids.
Another object of the invention is to provide an apparatus which can be used for precise quality control of liquids in industrial applications, and to provide an apparatus which can be portable for use in remote locations such as pipelines.
An additional object of the invention is to provide an apparatus with a relatively short cycle for providing data of temperature versus vapor pressure for a given liquid.